This invention relates to a device in the form of a pickup actuator for driving a pickup head movable in the direction of any one of three coordinate axes disposed perpendicular to one another in three-dimensional space.
In an optical video disc player and other optical reproducing apparatus using a beam of a laser, the beam of the laser is collected in a minuscule spot to detect a signal recorded on an information recording member. To read out a signal correctly requires effecting of focusing control for focusing the spot of light beam on the information recording member by accommodating the irregularities on the surface of the information recording member or vibration thereof and tracking control for causing the spot of light beam to follow up the correct signal track at all times. Also when there is the risk of a time base error being made due to variations in the rotation of the information recording member or eccentricity thereof, it is necessary to effect time base control to correct such error. To effect control as referred to hereinabove, it is essential that an actuator be provided which supports error sensing means for sensing errors and an optical system and operates such that it actuates the optical system on the basis of an output signal produced by the error sensing means so as to cancel out the error.
One type of actuator of the prior art is disclosed in Japanese Patent Application Laid-Open No. 146611/79, for example, in which a magnetic gap of annular shape and a magnetic gap perpendicular thereto are arranged in superposed relation and a drive coil is mounted in each of the magnetic gaps. This construction suffers the disadvantage that difficulties would be encountered in assembling the parts because it would be impossible to assemble a driven member after all the magnetic circuits have been assembled. Particularly, difficulties would be faced with in correctly positioning each drive coil in the associated magnetic gap. Also, since the magnetic gap of the annular shape is closed at one side, air currents would have difficultly in flowing into and out of the gap, thereby rendering it difficult to dissipate heat generated in the drive coil. The air trapped in the magnetic gap would tend to develop cavity resonance and the phase would change at resonance frequency, so that control would be effected with difficultly.
In the actuator of the prior art shown in Japanese Patent Application Laid-Open No. 146611/79 and other actuators known in the art, the support device used includes plate springs arranged in two stages which can be bent in one direction only. This construction has one disadvantage. Since a drive force oriented in the focusing direction is transmitted to the objective lens via the plate spring for effecting tracking, unnecessary resonance phenomenon would tend to develop. Also, when the driven member is driven in a tracking direction, the reaction occurring in the pivot of the tracking plate spring would be transmitted to the fixed portion through the focusing plate spring, thereby giving rise to resonance. Thus difficulties would be faced in effecting control.
Japanese Patent Application Laid-Open No. 109801/79 discloses another type of actuator in which the actuator is supported by a support device including eight L-shaped springs each formed by bending a rod-shaped spring at a right angle. Some disadvantages are associated with this construction. The springs of the L-shape would have one part of the L-shape twisted when the other part thereof is flexed. Because of this phenomenon, flexural vibration would coexist with torsional vibration and give rise to coupled vibration, so that a complex resonance phenomenon would take place. When the L-shaped springs are formed of metal material, the aforesaid resonance phenomenon would be intensified; when soft material is used for forming them, difficulties would be encountered in effecting stationary positioning.
Still another type of actuator including a mirror that can be tilted is commercially available in U.S.A. which is made by MCA company, Pioneer Company, etc., and used with a video disc player of the optical system. This type of actuator has already been exhibited publicly in Japan. In this type of actuator, the light beam of a laser would tend to be deflected due to the tilting of the mirror, thereby making it difficult to bring the light beam into coincidence with the optical axis of the optical system. This would make it necessary to use an objective lens of a large angle of view. This would increase the size and weight of the objective lens and render same more expensive. To drive a lens of a large size would require a drive system of the overall size.